Abstract: The purpose of this study was to compare scores on state standardised tests for five-year-old children in full- and half-day kindergarten programmes in the United States. Four thousand four hundred and eleven students participated in this three-year cohort study; each completed standards-based instruction in an urban public school setting. Students of poverty in full-day kindergarten programmes achieve significantly higher test scores in both English/Language Arts (+18.6 points) and Mathematics (+25.1 points) than students who participated in half-day kindergarten programmes. The significant difference in the scores of the two groups resulted in gains indicating an abatement of the student achievement gap that exists between students of poverty and students who are not impoverished. The calculated effects for students who came from poverty were subtracted from the average score for English/Language Arts (-22.6 points) and Mathematics (-23.2). The gain in scores attributed to full-day kindergarten amounted to almost what was lost from the effect of poverty.